The present invention relates to a keypad assembly having particular, but not exclusive, application in the field of portable radiotelephones.
Some driving forces behind the design of keypads are mechanical reliability, reliability of operation, reduced cost and efficiency of manufacture.
Keypads are used in the user interfaces of devices such as calculators and radiotelephones (e.g. mobile phones). One design of keypad is shown in cross-section in FIG. 1. There is a flexible rubber keymat 1 which has key portions 2 surrounded and joined to the main web of the keymat by resilient deformation zones 3. Each key portion has an upper region 4 which projects through a corresponding hole 5 in an outer housing 6 of the device, and a lower region 7 which terminates in a projection 8. The upper portion can include in-moulded or printed indicia 9 to allow a user to identify the key. When a user presses the upper region 4 of the key the resilience of the deformation zone surrounding the key allows the key to travel towards a switch assembly 10 associated with the key so that the projection 8 can engage the switch. When the key portion is released the deformation zone urges it to return to its original position as illustrated in FIG. 1. The switch assembly comprises an upper assembly 11, including a flexible dome 12 made of sheet material, and a lower assembly 13 comprising a circuit board 14 with exposed conductive tracks 15. When the key is pressed it urges the dome 12 to deform suddenly to the configuration illustrated at 12a or 12b, in which it bridges the conductive tracks 15 of the switch assembly and connects them together electrically. This connection is sensed by the device. When the key is released the dome returns to its natural state, disconnecting the tracks from each other. The sudden deformation of the dome provides a user with a pleasant tactile and/or aural feedback when a key is pressed.
Such keypads are widely used, but are expensive, and they suffer from the problem that the printing 9 on the surface of the key is prone to wear and tear and may become unreadable over time.
There has been an increased usage in recent times of so-called in-mould keymats. Specific types of in-mould keymats are termed film-on-plastic and film-on rubber, in reference to their constituent parts. These differ from the keymats described above in that the keymat starts as a planar sheet onto which the final markings of the keys, i.e. the text to appear thereon, is indelibly printed or etched. This has the advantage that the markings do not suffer so greatly from the wear and tear problems associated with more traditional rubber keymats.
The planar sheet is then selectively vacuum or heat formed in parts such that the keys are made to stand proud of the surrounding material. The spaces thus created behind the keys need to be filled with a solid material so that the keys may be used to exert pressure on a key dome 12, and hence actuate the switch 10, as in the previously described solution. This can be achieved in a number of ways, but a particularly suitable method uses injection moulding techniques to fill the spaces with a suitable solid material, such as rubber or plastics.
A problem with this solution is that if the rubber keymat is directly replaced by the in-mould variety, the keymat does not benefit from the resilient deformation zones 3. This is because the in-mould keymat is relatively rigid, and there is not so great a degree of mechanical independence between adjacent keys. This results in a tendency for pressure on a given key to drag down its neighbour, which can result in unwanted key presses. This in turn may lead to incorrectly dialled telephone numbers, for instance.
Another problem with the in-mould keymat is that the keystroke, i.e. the distance travelled by the key from its rest position to the point at which the circuit is completed, is only of the order of 0.2 mm. With traditional rubber keymat solutions, the compression of the rubber material and the resilient deformation zones contribute to a keystroke of approximately 1 mm. This distance feels pleasing to the user, whereas the 0.2 mm for the in-mould keymat gives something of a hair-trigger feel, and does not afford such a good tactile solution.
In addition, the two problems mentioned above can combine such that erroneous key presses can occur through pressure on the casing of the apparatus, particularly the front casing 6 of the apparatus, enclosing the keymat.